1. Field of the Invention
The present invention relates to the field of protection of the chains of insulators of high-voltage transmission lines.
2. Description of the Related Art
In high-voltage transmission lines, normally suspended to chains of insulators and anchored to tall support poles, the phenomenon of flashovers is known, which may arise particularly in critical environmental conditions—between the suspension means associated with the support pole, and hence grounded, and the transmission line conductors.
The corona effect is also known, produced by abnormal increments of the electric field. Such phenomenon is due to high electric loads and manifests itself on the first insulators of the chains, on the line side.
This is also the location where, in bundle lines, the supporting yoke of the conductors is connected to the last insulator by means of a metallic component which, despite its relatively small size, undergoes strong tensile stress.
In order to prevent the above-mentioned phenomena (flashovers and corona effect) from translating into corrosion of the metal parts next to the conductors, the use of so-called “guard rings” to be applied to the bottom of the chain of insulators has long been known. These guard rings have the function of improving the distribution of the electric load on the insulators, i.e. of reducing electrical field irregularities on the first insulators on the line side. As a result, by reducing the occurrence of electrical phenomena, the corrosive action thereof is reduced and any risk of early structural weakening, possibly leading to failure, is prevented.
Guard rings are generally manufactured from a rod or metal tube, bent so as to form a ring surrounding the lower end of the chain of insulators. These guard rings are further equipped with a support allowing the anchoring thereof directly on the spacing yoke. The support is suitably formed, so as to project also towards the insulators of the suspension chain.
It was also possible to ascertain that a guard ring must not necessarily fully surround the lower end of the chain of insulators: a protection projecting on the outer side of the line and of a suitable form, also called “racket”, is sufficient. Such protection, formed by a rod or by a tube bent bow-like which does not surround the entire chain of insulators, performs equally well the function of modifying the electric field shape and of moving away the flashover. In any case, whenever reference is made in the present description to a “guard ring”, it is intended to refer in general also to any type of suitably formed “racket-like protection”.
Although said phenomena (corona effect and flashovers) are well known as being damaging to chains of insulators, some high-voltage transmission lines so far lack guard rings or similar protections. For such plants the problem then arises of how to achieve protection thereof.
It is clear that the currently most readily available solution of this problem is that of detaching the conductors from their respective chain of insulators and of replacing the existing spacing yoke with another one equipped with a guard ring and hence suitably formed to house it. Such a solution, however, is not feasible due to at least 3 reasons:                in order to be able to replace the existing assembly, the alternative assembly, i.e. spacing yoke and guard ring, cannot be of just any shape: it must be designed so as to interface with the sockets existing on the chain. It is therefore necessary to design and manufacture ad hoc a plurality of alternative assemblies according to the features of the various types of existing chains, with evident significant costs;        the replacement of the existing spacing yoke with an alternative assembly implies the need, as said, to detach the line conductors from their respective chain, with evident, equally high, operating costs;        finally, the detachment operation of the conductors from the chain must necessarily be carried out on a de-energised transmission line. This is generally not acceptable due to the resulting discontinuing of the power supply which, considering that this operation must be repeated on multiple poles, is relatively prolonged.        